The present invention relates to data storage systems. More specifically, the present invention relates to data storage systems using read heads, which utilize multiple magnetic layers with differing magnetic orientations.
There is ever increasing demand for high data densities that require sensitive sensors to read data from a magnetic media. Advanced giant magnetoresistive (GMR) spin valve sensors that have increased sensitivity are replacing anisotropic magnetoresistive (AMR) sensors. A spin valve sensor consists of two soft magnetic layers separated by a thin conductive, non-magnetic spacer layer such as copper. An antiferromagnetic material (called the xe2x80x9cpinning layerxe2x80x9d) is placed adjacent to the first soft magnetic layer to prevent it from rotating. Antiferromagnetic materials exhibiting this property are termed xe2x80x9cpinning materialsxe2x80x9d. With its rotation inhibited, the first soft layer is termed the xe2x80x9cpinned layerxe2x80x9d. The second soft layer rotates freely in response to an external field and is called the xe2x80x9cfree layerxe2x80x9d. If the pinning layer is deposited before the free layer, the structure is called a xe2x80x9cbottom spin valvexe2x80x9d or xe2x80x9cBSVxe2x80x9d. The layers can also be deposited in reverse order with the pinning layer on the top, in which case it is called a xe2x80x9ctop spin valvexe2x80x9d or xe2x80x9cTSVxe2x80x9d.
The sensor must be stabilized against the formation of edge domain walls because domain wall motion results in electrical noise, which makes data recovery impossible. A common way to achieve this is with a permanent magnet abutted junction design. In this scheme, permanent magnets with high coercive field (i.e., hard magnets) are placed at each end of the sensor. The field from the permanent magnets stabilizes the sensor and prevents edge domain formation, as well as provides proper bias.
Abutted junctions are difficult to engineer for the following reasons. To properly stabilize the sensor, the magnet must provide more flux than can be absorbed by the free layer. This undesirable extra flux stiffens the free layer near the edge of the sensor. The junction must be carefully engineered to minimize this stray flux as well as the junction resistance. Also, a junction of dissimilar metals can cause unwanted strain in the sensor. The free layer will respond to the strain unless the magnetostriction is exactly zero. Another disadvantage is the nature of hard magnetic materials, which are multi-domained. Variation in domain size and shape lead to a distribution of domain coercivity. Lower coercivity domains may rotate when subjected to external fields. Such a domain near the sensor edge could cause domain wall formation in the active sensor and failure.
An alternative method of stabilization is to use an xe2x80x9cexchange tabxe2x80x9d design. In this case, the free layer is overlaid with a pinning material layer, which pins it in the proper direction. This layer is called an xe2x80x9cexchange tab layer,xe2x80x9d and it both protects against the formation of edge domains and helps bias the sensor properly. There are several advantages to the use of an exchange tab over abutted junction. There is no junction to produce stray magnetic flux or junction resistance. Also, the lack of a junction of abutted, dissimilar metals makes it less likely to produce high strain within the sensor.
The resistance of a spin valve sensor depends upon the relative angle between the magnetic moments of the free and pinned soft layers. To maximize the sensitivity and obtain a linear output signal, it is necessary to bias the free layer. An ideal bias condition is when the free layer is biased such that its magnetic moment is perpendicular to the magnetic moment of the pinned layer in the absence of an applied magnetic field. Since the pinned layer in the spin valve and outer portions of the free layer are preferably oriented perpendicular to each other, these magnetic orientations or pinning directions are typically established by separate thermal anneals, each in the presence of a differently oriented magnetic field. One method to achieve this is to choose pinning materials having differing blocking temperatures for pinning the pinned layer and for biasing the free layer. The pinning direction of the material with the higher blocking temperature is established first. A second anneal sets the pinning direction of the other material without affecting the first. A disadvantage of this approach is that there are few pinning materials with blocking temperature sufficiently high to use in a recording head. Rotation of the pinning direction can occur at temperatures near the blocking temperature, leading to long-term reliability issues. Use of a second material with lower blocking temperature reduces the sensor""s thermal stability, since the lowest blocking temperature determines the maximum useable temperature.
Methods of fabricating spin valve sensors in accordance with the invention include forming a pinning layer from an antiferromagnetic material and forming a synthetic antiferromagnet adjacent the pinning layer. A free ferromagnetic layer is formed, and exchange tabs are formed adjacent outer portions of the free ferromagnetic layer for biasing the free layer. The exchange tabs are formed from the same antiferromagnetic material as the first pinning layer. Then, the magnetic moments of the synthetic antiferromagnet are set, and the magnetic moment of the free ferromagnetic layer is biased, during a single anneal in the presence of a single magnetic field.